When using automatic cash dispensers, such as ATM machines or similar devices where a plastic card is utilized, the user must often use a four-digit number (PIN) known only to himself in order to receive authorization. There are, by far, however, not as many different PINs as there are users, which is why each PIN exists many times over.
The PINs may only contain decimal digits, to enable them to be entered using numerical keypads. In addition, they are not supposed to begin with a zero. This means that, given four digit positions, the result is a range of 9000 different PINS. The theoretically lowest probability of correctly guessing a PIN is, thus, 1/9000.